Real-Time Visualization of the Single-Nanoparticle Electrocatalytic Hydrogen Generation Process and Activity under Dark Field Microscopy

Gas Evolution in Water Electrolysis

Gas bubbles generated by the hydrogen evolution reaction and oxygen evolution reaction during water electrolysis influence the energy conversion efficiency of hydrogen production. Here, we survey what is known about the interaction of gas bubbles and electrode surfaces and the influence of gas evolution on practicable devices used for water electrolysis. We outline the physical processes occurring during the life cycle of a bubble, summarize techniques used to characterize gas evolution phenomena in situ and in practical device environments, and discuss ways that electrodes can be tailored to facilitate gas removal at high current densities. Lastly, we review efforts to model the behavior of individual gas bubbles and multiphase flows produced at gas-evolving electrodes. We conclude our review with a short summary of outstanding questions that could be answered by future efforts to characterize gas evolution in electrochemical device environments or by improved simulations of multiphase flows.

Wide-field imaging of active site distribution on semiconducting transition metal dichalcogenide nanosheets in electrocatalytic and photoelectrocatalytic processes

Semiconducting transition metal dichalcogenide (TMD) nanosheets are promising materials for electrocatalysis and photoelectrocatalysis. However, the existing analytical approaches are inadequate at comprehensively describing the operation of narrow-bandgap semiconductors in these two processes. Furthermore, the distribution of the reactive sites on the electrode surface and the dynamic movement of carriers within these semiconductors during the reactions remain ambiguous. To plug these knowledge gaps, an in situ widefield imaging technique was devised in this study to investigate the electron distribution in different types of TMDs; notably, the method permits high-spatiotemporal-resolution analyses of electron-induced metal-ion reduction reactions in both electrocatalysis and photoelectrocatalysis. The findings revealed a unique complementary distribution of the active sites on WSe2 nanosheets during the two different cathodic processes. Our facile imaging approach can provide insightful information on the heterogeneous structure-property relationship at the electrochemical interfaces, facilitating the rational design of high-performance electrocatalytic/photoelectrocatalytic materials.

Bridging Innovations of Phase Change Heat Transfer to Electrochemical Gas Evolution Reactions

Bubbles play a ubiquitous role in electrochemical gas evolution reactions. However, a mechanistic understanding of how bubbles affect the energy efficiency of electrochemical processes remains limited to date, impeding effective approaches to further boost the performance of gas evolution systems. From a perspective of the analogy between heat and mass transfer, bubbles in electrochemical gas evolution reactions exhibit highly similar dynamic behaviors to them in the liquid-vapor phase change. Recent developments of liquid-vapor phase change systems have substantially advanced the fundamental knowledge of bubbles, leading to unprecedented enhancement of heat transfer performance. In this Review, we aim to elucidate a promising opportunity of understanding bubble dynamics in electrochemical gas evolution reactions through a lens of phase change heat transfer. We first provide a background about key parallels between electrochemical gas evolution reactions and phase change heat transfer. Then, we discuss bubble dynamics in gas evolution systems across multiple length scales, with an emphasis on exciting research problems inspired by new insights gained from liquid-vapor phase change systems. Lastly, we review advances in engineered surfaces for manipulating bubbles to enhance heat and mass transfer, providing an outlook on the design of high-performance gas evolving electrodes.

Dual-Mode Imaging of Dynamic Interaction between Bubbles and Single Nanoplates during the Electrocatalytic Hydrogen Evolution Process

Gas bubble formation at electrochemical interfaces can significantly affect the efficiency and durability of electrocatalysts. However, obtaining comprehensive details on bubble evolution dynamics, particularly their dynamic interaction with high-performance structured electrocatalysts, poses a considerable challenge. Herein, dual-mode interference/total internal reflection fluorescence microscopy is introduced, which allows for the simultaneous capture of the evolution pathway of bubbles and the 3D motion of nanoplate electrocatalysts, providing high-resolution and accurate spatiotemporal information. During the hydrogen evolution reaction, the dynamics of hydrogen bubble generation and their interactions with single nanoplate electrocatalysts at the electrochemical interface are observed. The results unveiled that, under constant potential, bubbles initially manifest as fast-moving nanobubbles, transforming into stationary microbubbles subsequently. The morphology of stationary nanoplates regulates the trajectories of these moving nanobubbles while the pinned microbubbles induce the motion of the electrocatalysts. The dual-mode microscopy can be employed to scrutinize numerous multiphase electrochemical interactions with high spatiotemporal resolution, which can facilitate the rational design of high-performance electrocatalysts.

Electrophoretic Deposition Interferometric Scattering Mass Photometry

Interferometric scattering microscopy (iSCAT) has rapidly developed as a quantitative tool for the label-free detection of single macromolecules and nanoparticles. In practice, this measurement records the interferometric scattering signal of individual nanoparticles in solution as they land and stick on a coverslip, exhibiting an intensity that varies linearly with particle volume and an adsorption rate that reflects the solution-phase transport kinetics of the system. Together, such measurements provide a multidimensional gauge of the particle size and concentration in solution over time. However, the landing kinetics of particles in solution also manifest a measurement frequency limitation imposed by the slow long-range mobility of particle diffusion to the measurement interface. Here we introduce an effective means to overcome the inherent diffusion-controlled sampling limitation of spontaneous mass photometry. We term this methodology electrophoretic deposition interferometric scattering microscopy (EPD-iSCAT). This approach uses a coverslip supporting a conductive thin film of indium tin oxide (ITO). Charging this ITO film to a potential of around +1 V electrophoretically draws charged nanoparticles from solution and binds them in the focal plane of the microscope. Regulating this potential offers a direct means of controlling particle deposition. Thus, we find for a 0.1 nM solution of 50 nm polystyrene nanoparticles that the application of +1 V to an EPD-iSCAT coverslip assembly drives an electrophoretic deposition rate constant of 1.7 s-1 μm-2 nM-1. Removal of the potential causes deposition to cease. This user control of EPD-iSCAT affords a means to apply single-molecule mass photometry to monitor long-term changes in solution, owing to slow kinetic processes. In contrast with conventional coverslips chemically derivatized with charged thin films, EPD-iSCAT maintains a deposition rate that varies linearly with the bulk concentration.

Metal Core-Shell Nanoparticle Supercrystals: From Photoactivation of Hydrogen Evolution to Photocorrosion

Gas nanobubbles are directly linked to many important chemical reactions. While they can be detrimental to operational devices, they also reflect the local activity at the nanoscale. Here, supercrystals made of highly monodisperse Ag@Pt core-shell nanoparticles are first grown onto a solid support and fully characterized by electron microscopies and X-ray scattering. Supercrystals are then used as a plasmonic photocatalytic platform for triggering the hydrogen evolution reaction. The catalytic activity is measured operando at the single supercrystal level by high-resolution optical microscopy, which allows gas nanobubble nucleation to be probed at the early stage with high temporal resolution and the amount of gas molecules trapped inside them to be quantified. Finally, a correlative microscopy approach and high-resolution electron energy loss spectroscopy help to decipher the mechanisms at the origin of the local degradation of the supercrystals during catalysis, namely nanoscale erosion and corrosion.

Imaging Single H2 Nanobubbles Using Off-Axis Dark-Field Microscopy

A robust and detailed physicochemical description of electrochemically generated surface nanobubbles and their effects on electrochemical systems remains at large. Herein, we report the development and utilization of an off-axis, dark-field microscopy imaging tool for probing the dynamic process of generating single H2 nanobubbles at the surface of a carbon nanoelectrode. A change in the direction of the incident light is made to significantly reduce the intensity of the background light, which enables us to image both the nanoelectrode and nanobubble on the electrode surface or the metal nanoparticles in the vicinity of the electrode. The correlated electrochemical and optical response provides novel insights regarding bubble nucleation and dissolution on a nanoelectrode previously unattainable solely from its current-voltage response.

Bridging the Gap between Single Nanoparticle Imaging and Global Electrochemical Response by Correlative Microscopy Assisted By Machine Vision

The nanostructuration of an electrochemical interface dictates its micro- and macroscopic behavior. It is generally highly complex and often evolves under operating conditions. Electrochemistry at these nanostructurations can be imaged both operando and/or ex situ at the single nanoobject or nanoparticle (NP) level by diverse optical, electron, and local probe microscopy techniques. However, they only probe a tiny random fraction of interfaces that are by essence highly heterogeneous. Given the above background, correlative multimicroscopy strategy coupled to electrochemistry in a droplet cell provides a unique solution to gain mechanistic insights in electrocatalysis. To do so, a general machine-vision methodology is depicted enabling the automated local identification of various physical and chemical descriptors of NPs (size, composition, activity) obtained from multiple complementary operando and ex situ microscopy imaging of the electrode. These multifarious microscopically probed descriptors for each and all individual NPs are used to reconstruct the global electrochemical response. Herein the methodology unveils the competing processes involved in the electrocatalysis of hydrogen evolution reaction at nickel based NPs, showing that Ni metal activity is comparable to that of platinum.

In Situ Direct Monitoring of the Morphological Transformation of Single Au Nanostars Induced by Iodide through Dual-Laser Dark-Field Microscopy: Unexpected Mechanism and Sensing Applications

Single nanoparticle imaging is a significant technique to help reveal the reaction mechanism and provides insight into the nanoparticle transformation. Here, we monitor the in situ morphological transformation of Au nanostars (GNSs) induced by iodide (I⁻) in real time using dark-field microscopy (DFM) with 638 nm red (R) and 534 nm green (G) laser coillumination. The two lasers are selected because the longitudinal localized surface plasmon resonance of GNSs is located at 638 nm and that for GNSs after transformation is at 534 nm. Interestingly, I⁻ can interact with GNSs directly without the engagement of other reagents, and upon increasing I⁻ concentrations, GNSs undergo color changes from red to orange, yellow, and green under DFM. Accordingly, green/red channel intensities (G/R ratios) are extracted by obtaining red and green channel intensities of single nanoparticles to weigh the morphological changes and quantify I⁻. A single nanoparticle sensor is constructed for I⁻ detection with a detection limit of 6.9 nM. Finally, a novel mechanism is proposed to elucidate this shape transformation. I⁻ absorbed onto the surface of GNSs binds with Au atoms to form AuI⁻, lowering the energy of its bond with other Au atoms, which facilitates the diffusion of this atom across the nanoparticle surface to low-energy sites at the concaves, thus deforming to spherical Au nanoparticles.

Single-Particle Measurements of Nanocatalysis with Dark-Field Microscopy

Due to the complexity of heterogeneous reactions and heterogeneities of individual catalyst particles in size, morphology, and the surrounding medium, it is very important to characterize the structure of nanocatalysts and measure the reaction process of nanocatalysis at the single-particle level. Traditional ensemble measurements, however, only provide averaged results of billions of nanoparticles (NPs), which do not help reveal structure–activity relationships and may overlook a few NPs with high activity. The advent of dark-field microscopy (DFM) combined with plasmonic resonance Rayleigh scattering (PRRS) spectroscopy provides a powerful means for directly recording the localized surface plasmon resonance (LSPR) spectrum of single plasmonic nanoparticles (PNPs), which also enables quantitative measurements. In recent years, DFM has developed rapidly for a series of single-particle catalytic reactions such as redox reactions, electrocatalytic reactions, and DNAzyme catalysis, with the ability to monitor the catalytic reaction process in real time and reveal the catalytic mechanism. This review provides a comprehensive overview of the fundamental principles and practical applications of DFM in measuring various kinds of catalysis (including chemocatalysis, electrocatalysis, photocatalysis, and biocatalysis) at the single-particle level. Perspectives on the remaining challenges and future trends in this field are also proposed.

Single-Entity Electrochemistry of Nano- and Microbubbles in Electrolytic Gas Evolution

Gas bubbles are found in diverse electrochemical processes, ranging from electrolytic water splitting to chlor-alkali electrolysis, as well as photoelectrochemical processes. Understanding the intricate influence of bubble evolution on the electrode processes and mass transport is key to the rational design of efficient devices for electrolytic energy conversion and thus requires precise measurement and analysis of individual gas bubbles. In this Perspective, we review the latest advances in single-entity measurement of gas bubbles on electrodes, covering the approaches of voltammetric and galvanostatic studies based on nanoelectrodes, probing bubble evolution using scanning probe electrochemistry with spatial information, and monitoring the transient nature of nanobubble formation and dynamics with opto-electrochemical imaging. We emphasize the intrinsic and quantitative physicochemical interpretation of single gas bubbles from electrochemical data, highlighting the fundamental understanding of the heterogeneous nucleation, dynamic state of the three-phase boundary, and the correlation between electrolytic bubble dynamics and nanocatalyst activities. In addition, a brief discussion of future perspectives is presented.

Emerging Optical Microscopy Techniques for Electrochemistry

An optical microscope is probably the most intuitive, simple, and commonly used instrument to observe objects and discuss behaviors through images. Although the idea of imaging electrochemical processes operando by optical microscopy was initiated 40 years ago, it was not until significant progress was made in the last two decades in advanced optical microscopy or plasmonics that it could become a mainstream electroanalytical strategy. This review illustrates the potential of different optical microscopies to visualize and quantify local electrochemical processes with unprecedented temporal and spatial resolution (below the diffraction limit), up to the single object level with subnanoparticle or single-molecule sensitivity. Developed through optically and electrochemically active model systems, optical microscopy is now shifting to materials and configurations focused on real-world electrochemical applications.

Toehold-mediated strand displacement coupled with single nanoparticle dark-field microscopy imaging for ultrasensitive biosensing

Highly sensitive detection of biomarkers is essential for disease prevention and early diagnosis. Herein, a highly sensitive strategy was proposed for microRNA-21 (miRNA-21) detection by the incorporation of programmable toehold-mediated strand displacement (TMSD) and dark-field microscopy imaging. Firstly, efficient and specific TSMD was carried out via hybridization between the substrate strand (Sub) and two short probe strands (P1, P2). Then, miRNA-21 could specifically hybridize with Sub due to the toehold that existed on its tail, which triggered the amplification with the help of the assist strands, and forming a large number of Sub-assist double-stranded DNA (dsDNA). This process realized the targeted highly specific recognition of miRNA-21 and the amplification of the trace target to high-output dsDNA. Additionally, as glucose oxidase (Gox) was modified on the end of the assist strands in advance, hydrogen peroxide was generated after adding glucose to the system, which further etched gold-silver core-shell nanocubes (Au@Ag NCs). As a result, the size of Au@Ag NCs decreased and the scattering intensity reduced simultaneously. The scattering intensity reduction value of Au@Ag NCs has a linear relationship with miRNA-21 concentration in the range of 1.0 to 100.0 fM with a limit of detection of 1.0 fM. Finally, the proposed method has been successfully demonstrated for the determination of miRNA-21 in lung cancer cell A549 lysate.

An algorithm-assisted automated identification and enumeration system for sensitive hydrogen sulfide sensing under dark field microscopy

A sensitive H2S sensing strategy has been developed based on the automated identification and enumeration algorithm.

Plasmonic biosensor for the highly sensitive detection of microRNA-21 via the chemical etching of gold nanorods under a dark-field microscope

MicroRNAs involved in tumor-related tissues at abnormal expression level present tremendous potential in the early diagnosis of cancers. However, their intrinsic shortcomings, for instance, low abundance and high sequence homology, make it challengeable to quantify them with high sensitivity and selectivity. Herein, a highly sensitive platform with great specificity was developed for microRNA-21 based on the produced-I₂ triggered chemical etching of gold nanorods to a smaller size, resulting in a significant blue shift and a great intensity decrease in the localized surface plasmon resonance (LSPR) scattering. The synergism of strand displacement and enzymatic reaction enabled the proposed strategy with a high sensitivity and selectivity toward microRNA-21 in a dynamic range from 0.1 to 10,000 pM and a low limit of detection of 71.22 fM (3σ/k) by dark-field microscope. Additionally, the remarkable discrimination of single nucleotide difference suggested the superior selectivity towards microRNA-21, which presented a satisfactory recovery in human serum samples. The proposed plasmon platform could also serve as a universal and sensitive detection of cancer biomarkers, presenting the amusing application prospects in the early diagnosis of various cancers by adapting the corresponding nucleic acid sequences.

Single nanoparticle identification coupled with auto-identify algorithm for rapid and accurate detection of L-histidine

In this work, an auto-identify sensor was constructed for rapid and high-precision detection of L-histidine. The proposed strategy is based on the auto-identify algorithm and the aggregation of alkynyl and azide functionalized gold nanoparticles induced by the Cu⁺ catalyzed azides and alkynes cycloaddition (CuAAC) reaction. Specially, the color of scattering light spots for the aggregated gold nanoparticle (AuNPs) caused by CuAAC reaction was quite different from that of the monomers. However, L-histidine can bind to Cu²⁺ and inhibits the production of Cu⁺, hence preventing the aggregation of AuNPs. Therefore, there is a distinct change of color as the addition of L-histidine under dark-field microscopy. Then, L-histidine can be quantitatively detected by combining the color change with the Meanshift algorithm accurately and automatically. Such proposed method has been successfully applied for the detection of L-histidine in serum sample with satisfying result.

Nanobubble Labeling and Imaging with a Solvatochromic Fluorophore Nile Red

Herein, we report the use of a polarity-sensitive, solvatochromic fluorophore Nile red to label and probe individual hydrogen nanobubbles on the surface of an indium-tin oxide (ITO) electrode. Nanobubbles are generated from the reduction of water on ITO and fluorescently imaged from the transient adsorption and desorption process of single Nile red molecules at the nanobubble surface. The ability to label and fluorescently image individual nanobubbles with Nile red suggests that the gas/solution interface is hydrophobic in nature. Compared to the short labeling events using rhodamine fluorophores, Nile red-labeled events appear to be longer in duration, suggesting that Nile red has a higher affinity to the bubble surface. The stronger fluorophore-bubble interaction also leads to certain nanobubbles being co-labeled by multiple Nile red molecules, resulting in the observation of super-bright and long-lasting labeling events. Based on these interesting observations, we hypothesize that Nile red molecules may start clustering and form some kind of molecular aggregates when they are co-adsorbed on the same nanobubble surface. The ability to observe super-bright and long-lasting multifluorophore labeling events also allows us to verify the high stability and long lifetime of electrochemically generated surface nanobubbles.

Transpeptidation-mediated single-particle imaging assay for sensitive and specific detection of sortase with dark-field optical microscopy

Transpeptidation of surface proteins catalyzed by the transpeptidase sortase plays a critical role in the infection process of Gram-positive pathogen. Monitoring sortase activity and screening its inhibitors are of great significance to fundamental understanding of the infection mechanism and pharmaceutical development. Herein, we developed a digital single-particle imaging method to quantify sortase A (SrtA) activity based on transpeptidation-mediated assembly and enumeration of gold nanoparticles (GNPs). The assay utilizes two peptide stands, in which one has the SrtA recognition sequence LPXTG motif while the other carries an oligoglycine nucleophile at the one end and a biotin group at the other. The presence of SrtA enables the ligation of two peptides and allows for the immobilization of streptavidin-functionalized GNPs. Thus, SrtA activity can be quantified by imaging and enumeration of the surface-assembled GNPs at the single-particle level via dark-field microscopy. The single-particle method was highly sensitive to SrtA activity with a low detection limit of 7.9 pM and a wide linear dynamic range from 0.05 to 50 nM. Besides detection of SrtA in complex biological samples such as Gram-positive pathogen lysates, the proposed method was also successfully applied to estimate the half-maximal inhibitory concentration (IC₅₀) values of SrtA inhibitors (curcumin, berberine hydrochloride and quercetin). The present method, combining single-GNP counting and dark-field imaging, provides a facile and novel analytical tool for SrtA activity and its inhibitor screening.

Dynamic Equilibrium Model for Surface Nanobubbles in Electrochemistry

Gas bubbles are ubiquitous in electrochemical processes, particularly in water electrolysis. Due to the development of gas-evolving electrocatalysis and energy conversion technology, a deep understanding of gas bubble behaviors at the electrode surface is highly desirable. In this work, by combining theoretical analysis and molecular simulations, we study the behaviors of a single nanobubble electrogenerated at a nanoelectrode. With the dynamic equilibrium model, the stability criteria for stationary surface nanobubbles are established. We show theoretically that a slight change in either the gas solubility or solute concentration results in various nanobubble dynamic states at a nanoelectrode: contact line pinning in aqueous and ethylene glycol solutions, oscillation of pinning states in dimethyl sulfoxide, and mobile nanobubbles in methanol. The above complex nanobubble behavior at the electrode/electrolyte interface is explained by the competition between gas influx into the nanobubble and outflux from the nanobubble.

Imaging and Quantifying the Formation of Single Nanobubbles at Single Platinum Nanoparticles during the Hydrogen Evolution Reaction

While numerous efforts have been made toward the design of sustainable and efficient nanocatalysts of the hydrogen evolution reaction, there is a need for the operando observation and quantification of the formation of gas nanobubbles (NBs) involved in this electrochemical reaction. It is achieved herein through interference reflection microscopy coupled to electrochemistry and optical modeling. In addition to analyzing the geometry and growth rate of individual NBs at single nanocatalysts, the toolbox offered by superlocalization and quantitative label-free optical microscopy allows analyzing the geometry (contact angle and footprint with surface) of individual NBs and their growth rate. It turns out that, after a few seconds, NBs are steadily growing while they are fully covering the Pt nanoparticles that allowed their nucleation and their pinning on the electrode surface. It then raises relevant questions related to gas evolution catalysts, such as, for example, does the evaluation of NB growth at the single nanocatalyst really reflect its electrochemical activity?